This invention relates to spectrometers of the type which incorporate interferometers and use the Fourier Transform performed by a computer to convert electronic signals derived from the optical output of the interferometer into spectral analysis data.
Such interferometers generally incorporate three radiation sub-systems:
(1) the infrared radiation which is the basic analytical beam; PA1 (2) a monochromatic (laser) beam which derives pulses from a periodic fringe pattern to "clock" the sampling of detector signals by the computer system; and PA1 (3) a wide-band, or "white" light beam which is used to start each spectral scanning sweep at the identical point in the spectrum, in order that the integrated spectral data output will have maximum accuracy. PA1 (a) a moving retro-reflector in the variable-length arm which reflects both the analytical beam and the reference beam; PA1 (b) stationary reflecting means in the variable-length arm providing a flat "folding" reflector which causes the path of at least the reference beam from the retro-reflector to be folded on itself and returned to the retro-reflector; and PA1 (c) stationary reflecting means for the reference beam in the fixed-length arm so located as to offset the reference interferogram with respect to the analytical interferogram.
Alignment of the various interferometer optics and radiation beams requires extreme accuracy. One of the major problems is any undesired change of position of an optical element which has the effect of altering the relationship between the white light which produces the scanning reference point and the infrared light which produces the spectral analysis data. The correct relationship of the laser clocking beam to the other beams is also highly critical, but its arrangement is not the central feature of the present invention. The white light beam, which is itself passed through an interferometer, provides the synchronization pulse at some fixed point during the scan, in order to enable the data of successive scans to be coherently co-added. The central maximum of the interferogram provides a natural pulse for this purpose. However, in most systems, sampling must begin somewhat before the central maximum of the basic infrared spectral analysis interferogram. Therefore, it is necessary to obtain a displaced central maximum to use as a starting point reference. The usual means of accomplishing this displacement is to use a "secondary interferometer" which scans a white light signal during the same motion as the infrared scanning, but which is so located that the central maximum of the white light interferogram can be used to start the scan of the infrared interferogram. Generally, it is desirable to have the peak of the infrared interferogram near the center of the scan. In commercial instruments this is done in a variety of ways.
Interferometers in current use are susceptible to "shift" of the white light reference point (with respect to the infrared scan) under certain conditions. For example, certain instruments of Digilab use a small secondary interferometer, which is mounted back-to-back with a broad infrared interferometer, and which is adjusted to give the desired displaced white light central maximum. Since the two interferometers are back-to-back and use different mounting structures, the position of the displaced white light maximum is highly temperature dependent. In other words, temperature change can vary the relative path lengths of the white light and the infrared light, thereby shifting the starting point of the infrared scan. Certain instruments of Nicolet provide a separate white light interferometer which is mounted to one side of the infrared interferometer. If the moving element which carries the mirrors of both interferometers should tilt, such tilting will have its maximum effect on the position of the white light mirror, thereby giving rise to a significant shift of the white light reference central maximum relative to the infrared interferogram.
The present application is primarily concerned with improving the reliability of synchronization of the successive infrared scans by insuring against any displacement of the position where the white light, or reference, interferogram initiates the recorded infrared, or analytical, scan.